GB-A-2182892 discloses buoyancy apparatus for carrying a pay-load to and maintaining the pay-load at a predetermined depth. The apparatus described operates by sensing upward movement and downward movement of the buoyancy apparatus in water and injecting air into or venting air from a ballast tank to adjust the buoyancy of the device to maintain a constant depth.
It is also disclosed that such a device can be made to work using means for sensing movement in either the upward direction or the downward direction but not both. However, no specific apparatus working according to this principle is disclosed.